An HEV conventionally executes state of charge (SOC) control on a rechargeable battery to maximize fuel consumption efficiency of the vehicle by detecting voltage, current, temperature, and the like of the rechargeable battery and performing calculations to estimate the SOC of the rechargeable battery. For accurate execution of the SOC control, the SOC of the rechargeable battery during charging/discharging needs to be estimated correctly.
In one conventional method for estimating the SOC, a battery voltage V and a charge-discharge current I are measured over a predetermined period, an integrated value ∫I of the current is calculated, and a battery polarization voltage Vc(t−1), which was previously estimated, is updated to Vc(t) based on functions of the temperature T, the battery voltage V, and the integrated current value ∫I to obtain a correction voltage V′ (=V−Vc(t)). A plurality of data sets for the correction voltage V′ and current I are obtained and stored. A linear regression line (voltage V′-current I regression line) is obtained through regression analysis using the data sets. A V intercept of the V′-I regression line is estimated as an electromotive force E, and the SOC is estimated from functions of the previously estimated SOC, the electromotive force E, the temperature T, and the current integration value ∫I (refer, for example, to patent document 1).
Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-223033